The disclosures of Japanese Patent Application Nos. HEI 10-342023 filed on Dec. 1, 1998 and HEI 11-79803 filed on Mar. 24, 1999 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to a permanent magnet motor in which a rotor has a plurality of holes in which permanent magnets are disposed.
2. Description of the Related Art
In conventional permanent magnet motors, a plurality of permanent magnets are inserted and arranged in a rotor so that magnetic poles are formed. The rotor is driven by electrifying stator-side coils that are disposed substantially around the rotor and thereby forming rotating magnetic fields. Due to the permanent magnets, fields are formed individually for the predetermined poles, so that the motor size can be reduced and the motor output can be increased.
However, along with the pursuit for smaller sizes and faster operating speeds and higher outputs in motors, the problems of loss and heat generation due to eddy currents in the magnets become significant. Heat generation in a motor leads to demagnetization of the magnets, and may result in a failure of the motor. The losses caused by eddy currents reduce the motor efficiency.
Accordingly, it is an object of the present invention to provide a permanent magnet motor that reduces the loss and heat generation caused by eddy currents.
In accordance with one aspect of the invention, a permanent magnet motor includes a stator, and a rotor rotatably mounted in the stator. The rotor has a plurality of holes with a permanent magnet disposed in at least one of the holes. The permanent magnet is substantially divided into a plurality of permanent magnets arranged in a direction of a circumference of the rotor.
In the permanent magnet motor of the invention, a plurality of substantially divided permanent magnets are arranged in a hole in a circumferential direction. The divided arrangement reduces the total length of the outer peripheries of all the permanent magnets. Since eddy currents flow along an outer peripheral portion of each permanent magnet, the divided magnet arrangement increases the length of flow passages of eddy currents, so that the magnitude of eddy currents in each permanent magnet is reduced and the magnitude of eddy currents in all the permanent magnets is also reduced. Consequently, this permanent magnet motor reduces the eddy current loss and the heat generation of the motor.
The above-described permanent magnet motor may further have a construction in which the permanent magnet disposed in a hole is substantially divided into at least three divided permanent magnets, wherein one of the at least three divided permanent magnets that is located substantially in a middle of the hole is disposed radially outwardly, in a direction of a radius of the rotor, of the divided permanent magnets located at opposite ends of the hole.
By shifting the divided permanent magnets radially as described above, the arrangement of the divided permanent magnets becomes similar to an arcuate magnet. Therefore, if inexpensive flat or square type permanent magnets are used and shifted as described above, the permanent magnet motor can be produced at low costs while the torque ripple is substantially reduced.
At least one of the substantially divided permanent magnets in a hole may be formed by a ferrite magnet, and at least another one of the substantially divided permanent magnets may be formed by a rare-earth magnet. Since ferrite has high electric resistance, substantially no eddy current will occur in the ferrite magnet. Although the magnetic force of ferrite magnets is relatively weak, the reduction of the total magnetic force produced by all the permanent magnets can be curbed since only some of the permanent magnets are ferrite magnets and other magnets are rare-earth magnets.
Furthermore, a permanent magnet of the substantially divided permanent magnets that is located at a leading end of a hole with respect to a rotating direction of the rotor may be a ferrite magnet. Since the permanent magnet at the leading end of a hole enters a magnetic field produced by the stator, ahead of the other permanent magnets, the leading-end permanent magnet is more likely to experience greater eddy currents. However, placement of a ferrite magnet at the leading end position substantially eliminates eddy currents at that position, thereby enhancing the reduction of eddy currents.
In accordance another aspect of the invention, a permanent magnet motor includes a stator, and a rotor rotatable in the stator, the rotor having a plurality of holes. A permanent magnet is disposed in at least one of the holes. The permanent magnet is substantially divided into permanent magnet divisions by at least one slit in such a manner that adjacent ones of the permanent magnet divisions are interconnected at portions thereof.
Since the permanent magnet is substantially divided into permanent magnet divisions, eddy currents will be reduced. Furthermore, the permanent magnet divisions can be handled in the same manner as a single permanent magnet body. Therefore, this permanent magnet motor allows assembling operations and the like to be performed efficiently, and therefore improves the productivity.